Catechin, tannin, etc. are known as polyphenols contained in tea leaves. Particularly, catechins are known to have various physiological functions such as antioxidative action, antibacterial action, blood cholesterol-level-rise suppressive action, blood-pressure-rise suppressive action and blood-sugar-level-rise inhibitory action. However, catechins themselves are potent antioxidants and thus have a problem of undergoing oxidization and polymerization during storage. Further, catechins have a problem of discoloration upon oxidation and/or polymerization, thus restricting the degree of freedom of their application to pharmaceutical preparations. Accordingly, there is demand for a method of suppressing oxidation and polymerization of catechins.
JP-A 9-47654 relates to removal of polyphenols since the presence of excessive polyphenols in products deteriorates the commodity value, and is thus different in purpose from a method of suppressing oxidation and polymerization of catechins.
JP-A 8-333380 discloses a process for producing tea saponin, wherein an extract extracted with a water-containing lower alcohol from hot-water extraction residues of tea leaves is treated with water-insoluble polyvinyl pyrrolidone, thereby removing catechins from the extract.
JP-A 1-218550 discloses a process for producing tannin-free tea, which contains adding insoluble polyvinyl pyrrolidone to a filtered extract extracted with hot water, etc. from anaerobically treated tea leaves, and then filtering the extract.
It is known that dental caries and periodontitis are caused by dental bacteria. Accordingly, dental caries and periodontitis are prevented generally by killing dental bacteria or suppressing their activity with a dental composition compounded with a bacteriocide. Particularly with increasing natural-product orientation in recent years, a naturally derived component is used as the bacteriocide to avoid use of a chemically synthesized substance. For example, a green tea extract and its components that are tea polyphenol compounds (catechins etc.) are known to be effective in suppressing proliferation of dental bacteria (for example, JP-A 1-90124, JP-A 2-25413, JP-A 3-86814, etc.). These literatures describe that since green tea extracts, etc. prevent proliferation of Stereptococcus mutans that is a bacterium causing dental caries and Porphyromonas gingivalis that is a bacterium causing periodontitis, they can provide oral compositions effective in preventing dental caries and periodontitis. However, the polyphenol compounds (catechins etc.), when contacted with oxygen, undergo oxidation and polymerization to discolor significantly, so the discoloration with time of the oral composition itself compounded therewith is inevitable. As the method of suppressing such discoloration, for example, a method which contains incorporating sugar alcohols having 4 to 5 carbon atoms (for example, xylitol, erythritol etc.) into a polyphenol-containing plant extract and sealing the product in individual oxygen-impermeable bags is proposed (JP-A 2000-297022).
Meanwhile, as a method of incorporating various drugs, easily denatured active ingredients, and the like, into oral compositions, a method which contains stabilizing the ingredients by capsulating them or including them in shells etc. is also proposed (JP-A 61-225115, JP-A 1-275520, etc.).